Sift the flour, white sugar, baking soda, salt and 6 tablespoons of the cocoa into a mixing bowl. Add melted butter and vinegar and stir. Add the vanilla and cold water and stir well to remove lumps. Pour into a 9 x 13 inch baking pan.
Bake at 350 degrees F (175 degrees C) for 30 minutes. Spread with chocolate frosting when cool.
To Make Chocolate Frosting: Combine the confectioner's sugar, 6 tablespoons water, 3 tablespoons cocoa, and 1/2 cup of the butter. Heat until melted and stir until smooth. Remove from heat, stir in vanilla. Let sit until cake and frosting is cool, then frost.
Cake is best when allowed to sit several hours or overnite.